The present invention relates to an AGC (Automatic Gain Control) circuits in CDMA (Code Division Multiple Access) receivers.
As is well known in the art, the CDMA system is one of multiple access system techniques adopted in cellular systems (or mobile communication systems) such as car telephone systems and portable telephone systems, in which communication is performed simultaneously by a plurality of stations in the same frequency band. Well known as other multiple access techniques than the CDMA system are FDMA (Frequency Division Multiple Access) system and TDMA (Time Division Multiple Access) system. Over these other multiple access system techniques, the CDMA system has an advantage in higher frequency utilization efficiency and more user accommodation.
In the CDMA system, the spectrum of data signal to be transmitted is spread in a band sufficiently wide compared to the intrinsic data bandwidth, so that multiple connection is performed by spread spectrum communication.
The cellular system (or mobile communication system) generally includes mobile communication terminals (hereinafter referred to as “mobile stations”) MS and a plurality of base stations BS servicing cells. The CDMA system is employed as the third generation Mobile communication system. As hand-over in the case of cross-passing of a CDMA system mobile station MS from one cell to another, soft hand-over is executed, in which radio wave signal from the base station BS before the cross-passing and radio wave signal from the base station MS after the cross-passing are combined.
In such CDMA system, for increasing the subscriber's capacity a power control system is adopted, in which the SIR (signal-to-interference ratio) of receiver input is controlled to be constant. By the term “SIR” is meant the ratio of the level of desired wave received signal power to the level of interference signal power received by the pertinent user from a different user.
As power control system, Japanese Patent No. 2855173 (hereinafter referred to as Prior Art 1) proposes a “CDMA demodulating system”, in which the level of received power of pass after despreading is obtained by inserting a pilot signal of a known pattern in signal intermittently at a predetermined cycle frequency.
As further prior art techniques pertaining to the present invention, the following techniques are well known in the art. Japanese Patent Laid-Open No. 9-275361 (hereinafter referred to Prior Art 2) discloses receiver and transmitter, in which waveform distortion of even modulated waves having data in the amplitude is eliminated by instantly forming an AGC loop. In the receiver disclosed in Prior Art 2, a power amplifier amplifies a received signal by an antenna. A variable gain attenuator attenuates the gain of the received signal based on a holder circuit output voltage. A first mixer and a first local oscillator operate together to convert the output signal from the variable gain attenuator to an intermediate frequency signal, and a first band-pass filter limits the band of the intermediate frequency signal. A second local oscillator and a second mixer operate together for further frequency conversion of the first band-pass filter output, and a second band-pass filter limits the band. A limit amplifier limit-amplifies the second band-pass filter output signal, and its out-put is fed to a detector for detection. A comparator obtains a difference voltage between the detector output voltage and a reference voltage. When the difference voltage becomes positive, a timing generator generates a timing signal. Under control of the timing signal, the holder circuit holds the comparator output signal, and the variable gain attenuator attenuates the gain of the received signal based on the holder circuit output signal. A time constant circuit integrates the detector output, and an adder adds together the outputs of the time constant circuit and the holder circuit.
Japanese Patent Laid-Open No. 10-224293 (hereinafter referred to as Prior Art 3) discloses mobile station transmission power level control method and mobile communication system. Prior Art 3 concerns a problem in the prior art. Specifically, in the prior art mobile station transmission power level control, which is performed by a base station, is realized by step-wise control of control amount, which is fixed for each time slot. However, sometimes a long time is taken until the power level is converged to a predetermined level. In Prior Art 3, it is sought to prevent failure of convergence in a communication time particularly in burst communication. Specifically, the power level control executed in the first time slot from the start of communication is performed based on softly changing control amount, while the power level control in the second and following time slot is performed step-wise based on a fixed control amount.
Japanese Patent Disclosure No. 8-2058 (hereinafter referred to as Prior Art 4) discloses an AGC (automatic gain control) system, which is suited for fast and stable AGC in fading circumstances as in mobile communication, when applied to a direct conversion system a time division transmitting/receiving switching system. In Prior Art 3, the signal power level in burst section is detected by making use of a timing signal for time division transmission and reception, and gain control in the next reception time slot is performed based on the detection output. A gain control voltage is set at the start of received burst, and DC off-set can be removed by short-circuiting (or clamping) by short-circuiting the output of a DC block circuit capacitor to ground in a state that any received signal has not yet arrived. It is thus possible to completely remove the advantages of DC off-set variation due to AGC and improve the AGC speed.
However, Prior Art 1 has no mention about the control timing. Depending on the radio wave propagation circumstances, the line quality may be deteriorated by inadequate control timing, and in an extreme case the line may be broken.
Now, inconvenience which may be encountered when the control timing is inadequate will be described with reference to FIGS. 5(a) and 5(b).
It is important to control the control timing. Specifically, the result of calculation of average power level is desirably fed back from an instant corresponding to the forefront of the next slot.
In FIG. 5(a), two successive slots are shown. The first slot is referred to as slot 1, and the next slot is referred to slot 2. A predetermined period of time from the slot forefront is used for power level calculation. In the control shown in FIG. 5(a), the calculated power level in slot 1 is less than a desired level, and it is thus updated to the desired level from the instant corresponding to the forefront of slot 2. This control is ideal. However, such control is actually impossible.
Actually, as shown in FIG. 5(b), the control involves rising time. Accordingly, by taking the rise time into considerations the control is started at an instant earlier than the forefront of slot 2 with an aim of obtaining coincidence of the instant of reaching of a desired voltage with the forefront of slot 2. Control of signal 1 in FIG. 5(b) corresponds to this case.
The control is not started at an indiscriminately earlier instant for the following reason. The transmitting side transmits slots at a constant power level. Therefore, a change in the level of power inputted to the demodulator during a slot results in generation in an demodulation error or like undesired result. It is thus intended to minimize control change during the slot. This is the reason for the above.
However, the amount of control change (i.e., amount of correction) is not fixed. Now, inconvenience which may be encountered unless the control timing is changed will be described.
As an example, a propagation channel having a characteristic close to static characteristic with less fading effects will be considered. In this case, the time-wise change in the error of power level from desired level, i.e., the control amount, is not so much. Therefore, feed-back is provided at an instant earlier than desired instant. Waste is thus involved from the standpoint of minimizing the change in power level with time in one slot.
Now, a propagation channel with fading present therein will be considered. In this case, the received power level change with time may be great, i.e., the control amount may be extremely great. Therefore, feed-back is provided later than the desired instant. With this delay from the desired instant, the forefront of the slot used for the power level calculation occurs during the rise of voltage, thus disabling correct calculation. Signal 2 in FIG. 5(b) corresponds to this case.
What is shown above is the inconvenience encountered in the case of failure of adequate control of the control timing. Furthermore, in the case of using a known pilot signal for the power level calculation, the pilot signal may be too short to obtain correct average power level calculation.
Prior Art 2 shows an invention concerning a gain attenuator control circuit provided in a TDMA radio system, and it is thus set apart from the circuit applied to the CDMA system according to the present invention. Although according to Prior Art 2 a timing signal is generated, it is a signal synchronized to a received slot in the case where received signal peculiar to the TDMA system is a burst signal. That is, the timing signal disclosed in Prior Art 2 is quite irrelevant to the timing control of control amount, which is important according to the present invention.
According to Prior Art 3, time required from the start of transmission by a mobile station till convergence of the transmitted signal power level control is reduced by the power level control based on softly changing control amount in the first time slot from the start of communication. Specifically, In Prior Art 3 base station transmits transmission signal power level control signal of variable control amount to mobile station, and the mobile station controls the transmitted signal power level control in response to the transmitted signal power level control signal. That is, what is disclosed is merely reverse link power level control. Prior art 3 thus is essentially set apart from the control according to the present invention, in which the gain of signal received in own station is controlled to make the received signal power level constant.
According to Prior art 4, although the AGC system is disclosed, like the above Prior Art 2 the communication system as subject is a time division transmission/reception communication system, which is set apart from the CDMA system as the subject of the present invention. Although Prior Art 4 shows the use of timing signal, this signal is for time division transmission and reception, and gain control in the next time slot is performed based on an output obtained by burst section signal power level detection. Thus, the timing signal disclosed in Prior Art 4, like the above Prior Art 2, is quite irrelevant to the control timing of control amount as concerned by the present invention.